1. Field of the Invention
The present invention relates to a light emitting diode array used for light sources of an optical printer, a facsimile telegraph, etc. More particularly, the present invention relates to a light emitting diode array in which chip end faces are opposed to each other so as to arrange light emitting dots in a line.
2. Description of the Related Art
An electrophotographic system is widely used in a general digital office automation (OA) equipment such as a printer, a facsimile telegraph, etc. A laser diode and a light emitting diode array are used as a light source of the digital OA equipment. In particular, the light emitting diode array is suitable for the electrophotographic system operated at a high speed and having a wide width. The light emitting diode array is researched and developed as a light source of the optical printer having high performance.
FIG. 8 shows a general structure of the light emitting diode array. For example, in this structure, light emitting portions are composed of an n-type GaAs layer or an n-type AlGaAs layer 2 and a p-type GaAs layer or a p-type AlGaAs layer 3 are formed on an n-type GaAs substrate 1 so as to be spaced from each other at an equal distance.
A plurality of chips of the light emitting diode array having such a structure are arranged such that chip end faces are opposed to each other. Thus, a size of the light emitting diode array is increased by arranging the plural light emitting diode array chips in a line. For example, as shown in FIG. 9, when the light emitting diode array is fabricated as a light source of a printer, a plurality of chips of the light emitting diode array are adhered to each other by a conductive adhesive on a die bonding substrate 5 so as to increase the size of the light emitting diode array. In this case, the chips of the light emitting diode array must be arranged such that a clearance a between outside light emitting portions at separate chip ends is equal to a clearance b between inside light emitting portions within each of the chips so as not to reduce a printing quality between these chips. To arrange the chips in this way, a length c from an outside light emitting portion to a chip end portion must be equal to or smaller than a length half the clearance b between the inside light emitting portions.
Recently, the clearance b between the inside light emitting portions is very reduced as the light emitting diode array is fabricated with high density. In particular, it is necessary to set the clearance b between the inside light emitting portions to an especially small value so as to realize a light emitting diode array having a high density and a high output. In this case, the clearance b between the inside light emitting portions within each of the chips of the light emitting diode array can be reduced in principle until a length provided at an accuracy level of photolithography. However, it is very difficult to set the length c from the outside light emitting portion to the chip end portion to be equal to or smaller than a length half the clearance b between the inside light emitting portions since there is a mechanical limit with respect to a dieing technique for cutting the chips. In particular, in a compound semiconductor of GaAs, etc., a chipping portion having a size from several .mu.m to several ten .mu.m is caused at a dieing time of the chips. FIG. 10 is a cross-sectional view of an element end portion having a chipping portion 9 caused at the dieing time of a chip of the light emitting diode array at an end thereof. In FIG. 10, reference numeral 6 designates a cross section of the chipping portion 9 formed at the dieing time. To protect a light emitting portion from this chipping portion 9, a length c from the light emitting portion to a chip end portion must be set to be longer than a size of the chipping portion 9. This condition about the length c prevents a light emitting diode array having a high density from being realized.
In a light emitting diode array of an edge emitting type in which, a light emitting portion is composed of a light emitting diode of an edge emitting type, not only a densification of this diode array is prevented but also the uniformity of a light output is reduced in accordance with an accuracy in dieing and the generation of chipping. FIG. 11 shows a cross-sectional shape of a general light emitting diode array of an edge emitting type in a direction perpendicular to an arranging direction of light emitting diodes. In the light emitting diode array of an edge emitting type shown in FIG. 11, an n-type GaAs buffer layer 132 is formed on an n-type GaAs substrate 131. Further, an n-type Al.sub.0.4 Ga.sub.0.6 As clad layer 133, an Al.sub.0.2 Ga.sub.0.8 As active layer 134 and a p-type Al.sub.0.4 Ga.sub.0.6 As clad layer 135, so-called double hetero structure, are sequentially formed on the n-type GaAs buffer layer 132. A p-type GaAs cap layer 136 is further formed on the clad layer 135. Reference numerals 137 and 138 respectively designate a metallic electrode for injecting an electric current and an electrically insulated film. Reference numerals 6 and 8 respectively designate a cutting face of the substrate 131 cut by dieing and light emitted from a light emitting edge. A portion of this emitted light 8 is interrupted by a terrace-shaped portion 7 formed between the light emitting edge and a died portion so that this light portion cannot be used as a light output.
This terrace-shaped portion 7 in the general light emitting diode array of an edge emitting type is formed by dieing. Accordingly, there are dispersion in mechanical accuracy in dieing between chips and dispersion in cutting shape of each of the chips caused by chipping within each of the chips. Accordingly, no light output of a light emitting diode is uniformly influenced by the terrace-shaped portion 7 within a chip and a wafer. Therefore, the light output of the light emitting diode is dispersed by this terrace-shaped portion 7 even when the light output of the light emitting diode is uniformly provided in the light emitting diode array of an edge emitting type.
Accordingly, it is difficult to realize a light emitting diode array of an edge emitting type having a uniform light output so that such a light emitting diode array has not been practically used sufficiently.
Further, light of the light emitting diode array of an edge emitting type is emitted in a direction parallel to a substrate surface so that no light output can be measured in a wafer state. Therefore, the light output is measured with respect to each of separate elements after dieing. Hence, it takes much time and labor to measure the light output in this way so that no light emitting diode array of an edge emitting type has not been practically used sufficiently.